A typical heating, ventilating and air conditioning installation includes a heater for winter use, an air conditioner or cooler for summer use, a fan for drawing air across heater and cooler and discharging it to the space to be conditioned, a "return" air inlet from the space to be conditioned, an "outside" air inlet from the ambient atmosphere, a plenum for receiving air from the inlets, mixing it, and delivering it to the cooler, dampers in the inlets, means responsive to the temperature of the mixed air in the plenum for controlling the dampers to vary the ratio of outside to return air in the mixture, and means for venting the space to the atmosphere to prevent undesired pressure build-up within the space.
During the winter the temperature of the air discharged into the space from the installation is controlled by varying the proportions of outside, or cooler, air to return air, thermal input from the heater and fan operation being uniform. During the summer the temperature of the discharge is controlled in the same manner, except that the uniform thermal input is negative, from the cooler.
In the summer cycle it is customary to set the cooler in operation on the first warm day, and to leave the system in operation continuously for the entire season. The energy consumption for this form of operation is very considerable, and maximum wear of the cooling equipment is also inherent. There are, however, many occasions, particularly in the spring and fall, and often on summer mornings as well, when the outside air is at a sufficiently low temperature to supply all the cooling needed, so that the cost and energy drain of operating the cooler could be dispensed with. The situation is, of course, complicated by the fact that the humidity of the outside air may be too high for its use directly, without cooling to remove some of the moisture. Sensors have been developed for giving an output which is affected by both temperature and relative humidity, that is, by the "enthalpy" of the air ambient to the sensor. One suitable sensor can be identified as the Honeywell model H205A Enthalpy Control.